In the field of beverage producing machines, and in particular espresso makers, devices are known, which make use of capsules or pods, containing ingredients for the preparation of the beverage, for example ground coffee powder. The pod or capsule is introduced in a brewing chamber. The brewing chamber is closed and pressurized hot water is caused to flow through the capsule or pod to extract the coffee. An infusion unit using pods of this kind is disclosed for example in WO 00/49926. A hydraulic closure system is provided in this known device. The brewing chamber is formed by two substantially symmetrical brewing chamber portions which are opened and closed by the hydraulic system. Between the two brewing chamber portions a sealing gasket is provided, which seals the brewing chamber by squeezing the pod rim between the two brewing chamber portions when the brewing chamber is hydraulically closed.
In other known machines, ingredients for the preparation of the beverage are introduced in bulk form in the brewing chamber. For example, coffee beans are ground and the ground coffee powder is directly charged in the brewing chamber to subsequently perform an automatic brewing cycle. In other machines the coffee powder is manually charged in a filter placed in a filter holder, the filter holder is connected to a water dispensing unit and locked thereon; hot and pressurized brewing water is then delivered through the filter holder and the coffee bed pressed inside the brewing chamber.
An example of a manual machine using a filter holder for bulk material is disclosed in EP 2050370. An infusion unit having an automatic filter holder and brewing chamber is disclosed in WO 2009/157025.
In some known machines the dimension of the brewing chamber can be changed to brew either one or two cups of beverages. For example in manual machines different filter holders are used to house larger and smaller filters for producing alternatively two coffee cups or just one. In WO-A-2012/001592 a brewing unit for an automatic beverage-producing machine is disclosed, which is capable of modifying the volume of the brewing chamber based upon the amount of ingredients (e.g. ground coffee powder) loaded in the brewing chamber. An electric actuator moves a cup-shaped brewing chamber portion from a loading position to a brewing position. In the loading position coffee powder or another ingredient is loaded in the cup-shaped brewing chamber portion. In the brewing position the cup-shaped brewing chamber portion is moved towards a closure piston which enters into the cup-shaped brewing chamber portion and presses the ingredients therein. The piston is resiliently biased by a spring. The spring is compressed by the thrust exerted against the piston by the movable cup-shaped brewing chamber portion and by the ingredients loaded therein, under the control of the electric actuator. The piston is thus moved backwards as the cup-shaped brewing chamber portion moves towards a final brewing position. Once the cup-shaped brewing chamber portion has reached the final position, which is independent of the amount of ingredients loaded therein, the piston is hydraulically locked before starting the brewing cycle. The backwards stroke performed by the piston is dependent upon the amount of ingredients loaded in the brewing chamber. Movement of the piston is entirely passive, since the latter moves backwards under the action of the actuator that moves the cup-shaped brewing chamber portion forward.
Depending on the way of using the machine and/or on the quality of the ground coffee powder, a larger or smaller volume of coffee powder can be charged each time in the brewing chamber. This causes some difficulties when the brewing chamber is closed. If too much coffee powder is charged, the closing system is overstressed in case of an automatic machine. If the machine is of the manual kind, the operator has to exert a stronger force to close the brewing chamber. Especially in the case of manual machines, the amount of coffee powder can vary depending upon the ability of the operator and this can cause problems during brewing.
If an insufficient amount of coffee powder is loaded in the brewing chamber, a weak coffee is obtained and at the end of the brewing cycle a large amount of water remains on top of the exhausted coffee powder.
At the end of the brewing cycle a certain amount of water remains on top of the spent coffee powder. This amount of water is subsequently removed along with the spent coffee from the brewing chamber and increases the overall amount of residues generated by the brewing process. The energy used to heat such amount of water is not useful for the beverage production and is therefore wasted.
There is therefore a need for an improved infusion unit which overcomes or alleviates one or more the abovementioned drawbacks.